Automatic gain control (AGC) is an adaptive system found in many electronic devices. Typically, the average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels. AGC has been long-applied in receiver technologies.
For example, without AGC the sound emitted from an amplitude modulated (AM) receiver would vary to an extreme extent from a weak to a strong signal. The AGC effectively reduces the volume if the signal is strong and raises it when it is weaker. AGC technologies can also be applied, for example, to frequency modulated (FM) receivers and phase modulated (PM) receivers.
Digital receivers are designed to detect and amplify digitally encoded signals. Such receivers may also be paired with digital transmitters to form digital transceivers. In some instances, the digital receivers can be multi-mode, such as digital AM receivers and digital FM receivers. Various AGC systems have been designed for such digital receivers.
Since AGCs operate by feeding back some of the output of a stage to control the gain, they are often referred to as having “AGC loops.” AGCs are of three general types: 1) all-digital; 2) analog; and 3) mixed-mode including both digital and analog components.
The output of digital receivers is typically not ready to be input into a digital processing device such as a microprocessor. The output must be first digitized in a digitizer stage. A common form of digitizer is known as a “data slicer.” A data slicer works, in a way, like the demodulator portion of a modem. That is, a data slicer takes audio signals derived from a radio signal and converts them into digital bits (i.e. an analog to digital conversion). A stream of these digital bits, often arranged as “words”, can be input into a processor for digital processing. Data slicers decode FSK (Frequency Shift Keying) signals and ASK (Amplitude Shift Keying) signals.
In U.S. Pat. No. 5,933,455 of Hendrickson et al. an adaptive R/C circuit to improve threshold attack time is described. While accurate, adaptive R/C circuits have a rather slow acquisition time, resulting in possible data loss.
In U.S. Pat. No. 6,680,984 of Teghararian et al. a mixed-mode threshold generator is described that allows the receiver to be disabled and re-enabled while maintaining a proper slicing threshold during the sleep interval. However, the design requires a complex and expensive mixed-mode loop to preserve the proper slicing threshold during a sleep cycle.
In U.S. Pat. No. 6,735,260 Elizer et al. describe a threshold generator that uses dual peak detectors with an adaptable time constant to improve acquisition time. The peak detectors remain on after acquisition, resulting in a continuous current drain and the addition of noise to the output signal. U.S. Pat. No. 7,266,163 of Khorram et al. discuss a similar adaptive threshold generator.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.